1. Field of the Invention
The present invention relates to a method and an apparatus for measuring absorption information of a scattering medium and, more particularly, to a method and apparatus for measuring a temporal change or a spatial distribution of concentration of an absorptive constituent in a scattering medium having non-reentrant surfaces. The invention further concerns a method and apparatus for measuring a concentration of an absorptive constituent inside the scattering medium by use of light of plural wavelengths.
2. Related Background Art
There are very strong demands for non-invasive and precise measurements of absorption information including a concentration of a specific absorptive constituent inside a scattering medium like a living body, a temporal change or a spatial distribution thereof, and so on. Attempts of various methods have been made heretofore, including methods using continuous wave light (cw light) and modulated light (for example, pulsed light, square-wave light, sine-wave modulated light, etc.), methods utilizing light of different wavelengths (multi-wavelength spectroscopy), and so on.
These conventional technologies, however, are not yet capable of accurately measuring the concentration of the specific absorptive constituent inside for tissues and organs having various shapes like the living body or for objects having individual differences of shape though being tissues or organs of a same kind. This presents a serious problem for non-invasive measurements of living body utilizing light, and improvements therein are strongly desired.
Light incident to the scattering medium like the living body propagates inside is scattered and absorbed therein, and then part of the light emerges from its surface. Since the outside of the scattering medium is normally air, the light emerging from the surface is dispersed in the free space. The light emerging from the surface as described above is detected in measurements of internal information of scattering medium. At this time the propagating light spreads throughout the entire region of scattering medium and is dispersed from the whole surface to the outside. Therefore, when the output light is detected at a specific position in the surface, the quantity or a time-resolved waveform of detected light greatly varies with change in the shape of medium, for example, depending upon whether it is a sphere or a rectangular parallelepiped.
In order to enhance the measurement accuracy in the cases as described above, it is necessary to sufficiently understand the behavior of light inside the scattering medium. Recently, the behavior of light inside the scattering medium has been analyzed, tested, or investigated by Monte Carlo simulations with a computer. It is also known that the behavior can be described and analyzed accurately to some extent by the photon diffusion theory. The Monte Carlo simulations, however, require an extremely long calculation time and do not allow calculation of a concentration of a specific absorptive constituent inside the scattering medium from their results. In utilizing the photon diffusion theory, it is necessary to set boundary conditions for solving the photon diffusion equation. However, since the boundary conditions differ greatly depending upon the shape of scattering medium, new boundary conditions must be set to solve the photon diffusion equation for every change in the shape of scattering medium, in order to achieve accurate measurement. Scattering media for which the boundary conditions can be set accurately to some extent are limited to very simple shapes such as an infinite space, a semi-infinite space, an infinite cylinder, or a slab spreading infinitely and having a finite thickness. As a result, use of approximate boundary conditions is indispensable to measurements of living tissues having complicated shapes, which is a cause to produce large measuring errors.
The above problems are also discussed, for example, in the recent literature: Albert Cerussi et al., "The Frequency Domain Multi-Distance Method in the Presence of Curved Boundaries," in Biomedical Optical Spectroscopy and Diagnostics, 1996, Technical Digest (Optical Society of America, Washington DC, 1996) pp. 24-26.
As described above, there are no methods for measuring absorption information sufficient to be systematically applied to scattering media of different shapes, and it was extremely difficult for conventional technologies to systematically accurately and efficiently measure the concentration of a specific absorptive constituent or the like in the scattering media of different shapes.